117 Cardiac work (CW)

Cardiac work and cardiac power output: formula, typical resting value and the 0.53 W threshold in cardiogenic shock.

Cardiac work (cardiac minute work, or cardiac power) describes the rate at which the heart performs hydraulic work: it pumps a volume of blood per minute against arterial pressure. It equals the product of cardiac output and mean arterial pressure, and in power units it is called cardiac power output (CPO). Unlike ejection fraction or cardiac output alone, it takes account of the afterload and so reflects the true pumping capacity of the heart and the interaction between ventricle and arteries. In cardiogenic shock, cardiac power was the strongest haemodynamic predictor of in-hospital mortality.

HaemodynamicsHeart failureCardiogenic shock

FormulaCPO (W) = MAP × CO / 451

Healthy adult at rest≈ 1 W (calculated)

High risk in shock≤ 0.53 W

Equivalentstroke work × heart rate

Figure 1. Cardiac power output. Example: MAP 93 mmHg × CO 5 L/min / 451 ≈ 1.0 W. The 0.53 W threshold comes from the SHOCK trial registry.

Calculation

Cardiac power output (W) = mean arterial pressure (mmHg) × cardiac output (L/min) / 451

The constant 451 converts mmHg·L/min into watts. Cardiac work can also be expressed as stroke work (stroke volume × pressure) multiplied by heart rate. Cardiac output for the calculation is measured by thermodilution, the Fick method or echocardiography. During exercise, cardiac power rises several times; the maximum reflects the heart's reserve.

Clinical significance

Low cardiac work occurs in heart failure with reduced pumping function, after large myocardial infarction, in cardiomyopathy and especially in cardiogenic shock, where low cardiac power is strongly associated with death. In chronic heart failure, low resting and peak cardiac power is associated with worse outcomes. Cardiac power is used to guide decisions on medicines that support the heart and on mechanical circulatory support.

Seek emergency care

for breathlessness at rest, chest pain, fainting, cold clammy skin or confusion.

References

Fincke R, Hochman JS, Lowe AM, et al. Cardiac power is the strongest hemodynamic correlate of mortality in cardiogenic shock: a report from the SHOCK trial registry. J Am Coll Cardiol. 2004;44(2):340–348. doi:10.1016/j.jacc.2004.03.060
Vincent JL. Understanding cardiac output. Crit Care. 2008;12(4):174. doi:10.1186/cc6975

This article is for general information and does not replace a consultation with a doctor. Measurements must be interpreted by a physician together with symptoms and other tests.

Published on 8 May 2024